The present invention generally relates to an electrochemical cell, and more particularly relates to an improved anode for an alkaline cell having large particle gelling agent material disposed in the anode.
Conventional alkaline cells commonly use a gelled negative electrode, commonly referred to as the anode, which contains carboxymethyl cellulose (CMC), cross-linking type branched polyacrylic acid or a sodium salt thereof, natural gum, or the like as a gelling agent. The gelled anode is commonly formed by uniformly dispersing zinc powder in a gelled electrolyte, with the CMC as a gelling agent. The gelled anode typically retains a good gel state initially, however, with the passage of time, the CMC generally undergoes syneresis, and the anode falls out of the gel state. Also, the zinc powders, which have a large specific gravity, precipitate to reduce the effective surface area for reaction of the zinc. As a consequence, the alkaline cell may suffer a reduction in discharge performance.
To further add to the above-identified problem, when a cell is subjected to strong shock, e.g., when the cell is dropped or strong vibration is exerted thereon, the shock can cause the contact among zinc particles dispersed in the gelled electrolyte and/or that between the zinc particles and the anode current collector to become unstable. As a result, the electricity production effect of the cell may become unstable or incomplete.
One approach to preventing the above-identified problems has been to increase the volume content of the zinc powder in the gelled anode so that the degree of contact among the zinc particles and the collector is increased. When this is done, however, it is also necessary to increase the zinc powder volume considerably. Yet, if the concentration of zinc particles is too high, the packing of the zinc oxide and hydroxide product of discharge is so high as to block off ion diffusion. Also, if the zinc per cell is too high, the volume balance between the anode and cathode is lost, which can lead to a high gassing on deep discharge of the cell.
According to another approach, it is generally known to use a plurality of large particle nuggets uniformly dispersed throughout the anode mix of an alkaline cell so as to consume anode volume and have the effect of increasing zinc powder concentration in those portions of the anode that are not occupied by the nuggets. One example of the use of large particle nuggets is disclosed in U.S. Pat. No. 3,884,721, which is hereby incorporated by reference. The cell disclosed in the aforementioned issued patent employs a cross-linked polyacrylamide that absorbs alkaline electrolyte to form "electrolyte nuggets." With this approach, both the electrolyte nuggets and the zinc powder are distributed evenly throughout the anode so as to realize an increase in the zinc concentration in the non-nugget regions and an increase in the permeability of gas in the anode without exceeding the desired maximum of zinc per cell. Yet, the need to find new ways to increase service performance remains the primary goal of the cell designers.